What they worth? Are they any good? Would I get enough for them to pay for some nice shelf speakers?

I cant find much online about them…
Mrs S bought them 20 years ago ish.

Just looked on eBay completed listings; they show a pair went for £26.66 a few weeks ago. These were (perhaps, looking at them) oak effect – suspect that black ash, whilst very popular in the 80's and 90's, may not be so popular now.

hmm. Cheers HH.
Not going to able to retire just yet then…? 😉

EDIT: Just found those eBay ones and theyre "M20"s.

Still not found anything about the "S" range 🙁

I sold some Mission 760i's, from the late 80's, several months ago – recall postage being similar to the selling price…

Might be worth asking on an audio/hi-fi geek forum?

Most "audio/hifi geek" forums as you put are are the domain of the Naim/Linn members so doubt you'd get much help there. Much as they talk of the bike site geeks subcribing to them being the domain of On One/SS owners lol.
Best bet is just stick them on fleabay and see how it goes. They aren't desirebale in todays market with audiophiles so won't hanker after them but there'as always a buyer for stuff on the bay. Something is better than nothing I guess and will give you some money for some Wharfedale Diamonds 🙂

When you do get shot be sure to get some TANNOY M2 shelf speakers they are the furkin puppies nuts!